1. Field of the Invention
This invention relates generally to perpendicular magnetic recording media, and more particularly to a “dual-layer” perpendicular magnetic recording disk with a recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL).
2. Description of the Related Art
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular or out-of-plane orientation in the recording layer, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives. A common type of perpendicular magnetic recording system is one that uses a “dual-layer” media. This type of system is shown in FIG. 1 with a single write pole type of recording head. The dual-layer media includes a perpendicular magnetic data recording layer (RL) formed on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL). The SUL serves as a flux return path for the field from the write pole to the return pole of the recording head. In FIG. 1, the RL is illustrated with perpendicularly recorded or magnetized regions, with adjacent regions having opposite magnetization directions, as represented by the arrows. The magnetic transitions between adjacent oppositely-directed magnetized regions are detectable by the read element or head as the recorded bits.
FIG. 2 is a schematic of a cross-section of a prior art perpendicular magnetic recording disk showing the write field HW acting on the recording layer RL. The disk also includes the hard disk substrate, an adhesion or onset layer (OL) for growth of the SUL, a nonmagnetic exchange break layer (EBL) to break the magnetic exchange coupling between the magnetically permeable films of the SUL and the RL and to facilitate epitaxial growth of the RL, and a protective overcoat (OC). As shown in FIG. 2, the RL is located inside the gap of the “apparent” recording head (ARH), which allows for significantly higher write fields compared to longitudinal or in-plane recording. The ARH comprises the write pole (FIG. 1) which is the real write head (RWH) above the disk, and an effective secondary write pole (SWP) beneath the RL. The SWP is facilitated by the SUL, which is decoupled from the RL by the EBL and by virtue of its high permeability produces a magnetic mirror image of the RWH during the write process. This effectively brings the RL into the gap of the ARH and allows for a large write field HW inside the RL.
One type of material for the RL is a granular ferromagnetic cobalt alloy, such as a CoPtCr alloy, with a hexagonal-close-packed (hcp) crystalline structure having the c-axis oriented substantially out-of-plane or perpendicular to the RL. The granular cobalt alloy RL should also have a well-isolated fine-grain structure to produce a high-coercivity (Hc) media and to reduce inter-granular exchange coupling, which is responsible for high intrinsic media noise. Thus, the RL may include oxides, such as oxides of Si, Ta and Nb, which precipitate to the grain boundaries to enhance the grain segregation in the cobalt alloy RL. A perpendicular magnetic recording medium with a RL of a CoPtCr granular alloy with added SiO2 is described by H. Uwazumi, et al., “CoPtCr—SiO2 Granular Media for High-Density Perpendicular Recording”, IEEE Transactions on Magnetics, Vol. 39, No. 4, July 2003, pp. 1914-1918. A perpendicular magnetic recording medium with a RL of a CoPt granular alloy with added Ta2O5 is described by T. Chiba et al., “Structure and magnetic properties of Co—Pt—Ta2O5 film for perpendicular magnetic recording media”, Journal of Magnetism and Magnetic Materials, Vol. 287, February 2005, pp. 167-171.
The SUL is typically comprised of a high moment (Ms) material with a total thickness in the range of about 50 nm to 400 μm. The Ms and thickness of the SUL must be sufficient to avoid SUL saturation, which will deteriorate the writing performance. The critical thickness t(SUL) should satisfy the following equation:t(SUL)=Ms(Write Head)/2Ms(SUL)×LW/(L+W),where L and W are the length and width of the write pole, respectively. To achieve higher recording densities on the disk, the write pole will be made smaller, which will allow the thickness of the SUL to decrease. A high Ms for the SUL is required to prevent a degradation of the write field and the write-field gradient, which would decrease recording performance. However, a higher Ms for the SUL material will also reduce the SUL thickness requirement. Because the SUL is typically formed of amorphous magnetically permeable materials that include Co, Fe, and/or Ni, which are very reactive and easily form oxides and nitrides when exposed to air or water, the SUL is highly susceptible to corrosion. Thus, it is important that the SUL not only have a high Ms, but also be resistant to corrosion, especially for thinner SULs.
CoTaZr and CoNbZr alloys are known materials for use as the SUL. However, these high-moment alloys have poor corrosion resistance.
What is needed is a perpendicular magnetic recording medium with a SUL that achieves good corrosion resistance and high Ms, so that the recording properties of the medium are not degraded.